The present invention relates to a sealant or potting formulation prepared from a mercapto-terminated polymer produced by the reaction of polythiol(s) and polyvinyl ether monomer(s), the formulation having good low temperature flexibility and fuel resistance.
Commercially available polymeric materials which have sufficient sulfur content to exhibit desirable sealing and fuel resistance properties for aerospace sealants and electrical potting compounds are the polysulfide polyformal polymers described, e.g., in U.S. Pat. No. 2,466,963, and the alkyl side chain containing polythioether polyether polymers described, e.g., in U.S. Pat. No. 4,366,307 to Singh et al. Materials useful in this context also have the desirable properties of low temperature flexibility characterized by a low glass transition temperature (Tg) and liquidity at room temperature.
An additional desirable combination of properties for aerospace sealants which is much more difficult to obtain is the combination of long application time (i.e., the time during which the sealant remains usable) and short curing time (the time required to reach a predetermined strength). Singh et al., U.S. Pat. No. 4,366,307, disclose such materials. Singh et al. teach the acid-catalyzed condensation of hydroxyl-functional thioethers. The hydroxyl groups are in the xcex2-position with respect to a sulfur atom for increased condensation reactivity. The Singh et al. patent also teaches the use of such hydroxyl-functional thioethers with pendant methyl groups to afford polymers having good flexibility and liquidity. However, the disclosed condensation reaction has a maximum yield of about 75% of the desired condensation product. Furthermore, the acid-catalyzed reaction of xcex2-hydroxysulfide monomers yields significant quantities of an aqueous solution of thermally stable and highly malodorous cyclic byproducts, such as 1-thia-4-oxa-cyclohexane which limits the suitable application of the disclosed polymers.
Another desirable feature in polymers suitable for use in aerospace sealants is high temperature resistance. While incorporating sulfur to carbon bonds into a polymer generally enhances high temperature performance, the polysulfide polyformal polymers disclosed in U.S. Pat. No. 2,466,963 have multiple xe2x80x94Sxe2x80x94Sxe2x80x94 linkages in the polymer backbones which result in compromised thermal resistance. In the polymers of Singh et al., U.S. Pat. No. 4,366,307, enhanced thermal stability is achieved through replacement of polysulfide linkages with polythioether (xe2x80x94Sxe2x80x94) linkages. However, the thermal resistance of these polythioethers is limited as a result of residual acid condensation catalyst.
Morris et al., U.S. Pat. No. 4,609,762, describes reacting dithiols with secondary or tertiary alcohols to afford liquid polythioethers having no oxygen in the polymeric backbone. Cured polymeric materials formed from these polymers have the disadvantage, however, of reduced fuel resistance due to the large number of pendant methyl groups that are present. In addition, the disclosed process generates undesirable aqueous acidic waste.
Cameron, U.S. Pat. No. 5,225,472, discloses production of polythioether polymers by the acid-catalyzed condensation of dithiols with active carbonyl compounds such as HCOOH. Again, this process generates undesirable aqueous acidic waste.
The addition polymerization of aromatic or aliphatic dithiols with diene monomers has been described in the literature. See, e.g., Klemm, E. et al., J. Macromol. Sci.-Chem., A28(9), pp. 875-883 (1991); Nuyken, O. et al., Makromol. Chem., Rapid Commun. 11, 365-373 (1990). However, neither Klemm et al. nor Nuyken suggest selection of particular starting materials to form a polymer that is liquid at room temperature and, upon curing, has excellent low-temperature flexibility (low Tg) and high resistance to fuels, i.e., hydrocarbon fluids. Nor do Klemm et al. suggest production of a polymer that also is curable at room or lower temperatures. Moreover, the reactions disclosed by Klemm et al. also generate undesirable cyclic byproducts.
There exists a need in the art for sealant, coating and electrical potting formulations or compositions that can provide good pot life as well as good performance properties, such as fuel resistance, flexural strength, thermal resistance and longevity in use.
The present invention provides a sealant or potting formulation prepared from components comprising (a) at least one ungelled mercapto-terminated polymer prepared by reacting reactants comprising at least one polyvinyl ether monomer and at least one polythiol material; (b) at least one curing agent reactive with a mercapto group of the mercapto-terminated polymer; and (c) at least one additive selected from the group consisting of fillers, adhesion promoters, plasticizers and catalysts.
Another aspect of the present invention is a sealant or potting formulation prepared from components comprising: (a) at least one ungelled mercapto-terminated polymer prepared from reactants comprising diethylene glycol divinyl ether and dimercapto dioxaoctane; (b) at least one curing agent reactive with a mercapto group of the mercapto-terminated polymer; and (c) at least one additive selected from the group consisting of fillers, adhesion promoters, plasticizers and catalysts.
The above sealant formulations are useful in a variety of applications, such as for example aerospace applications or as electrical potting compounds.
Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being qualified in all instances by the term xe2x80x9caboutxe2x80x9d. Also, as used herein, the term xe2x80x9cpolymerxe2x80x9d is meant to refer to oligomers, homopolymers and copolymers.